Systems and methods for delivering therapy to the dorsal horn of a patient

ABSTRACT

A method of operating an implantable neuromodulator coupled to an electrode array implanted adjacent tissue of a patient having a medical condition comprises conveying electrical modulation energy to tissue of the patient in accordance with a modulation parameter set, wherein conveying the electrical modulation energy to tissue of the patient in accordance with the modulation parameter set stimulates dorsal horn neuronal elements more than dorsal column neuronal elements.

CLAIM OF PRIORITY

This application is a continuation of U.S. application Ser. No.14/556,742, filed Dec. 1, 2014, which claims the benefit of priorityunder 35 U.S.C. § 119(e) of U.S. Provisional Patent Application Ser. No.61/911,728, filed on Dec. 4, 2013, each of which is herein incorporatedby reference in its entirety.

FIELD OF THE INVENTION

The present inventions relate to tissue modulation systems, and moreparticularly, to programmable neuromodulation systems.

BACKGROUND OF THE INVENTION

Implantable neuromodulation systems have proven therapeutic in a widevariety of diseases and disorders. Pacemakers and Implantable CardiacDefibrillators (ICDs) have proven highly effective in the treatment of anumber of cardiac conditions (e.g., arrhythmias). Spinal CordStimulation (SCS) systems have long been accepted as a therapeuticmodality for the treatment of chronic pain syndromes, and theapplication of tissue stimulation has begun to expand to additionalapplications such as angina pectoralis and incontinence. Deep BrainStimulation (DBS) has also been applied therapeutically for well over adecade for the treatment of refractory chronic pain syndromes, and DBShas also recently been applied in additional areas such as movementdisorders and epilepsy. Further, in recent investigations, PeripheralNerve Stimulation (PNS) systems have demonstrated efficacy in thetreatment of chronic pain syndromes and incontinence, and a number ofadditional applications are currently under investigation. Furthermore,Functional Electrical Stimulation (FES) systems, such as the Freehandsystem by NeuroControl (Cleveland, Ohio), have been applied to restoresome functionality to paralyzed extremities in spinal cord injurypatients.

These implantable neuromodulation systems typically include one or moreelectrodes carrying stimulation leads, which are implanted at thedesired stimulation site, and an implantable neuromodulation device(e.g., an implantable pulse generator (IPG)) implanted remotely from thestimulation site, but coupled either directly to the neuromodulationlead(s) or indirectly to the neuromodulation lead(s) via a leadextension. The neuromodulation system may further comprise a handheldexternal control device (e.g., a remote control (RC)) to remotelyinstruct the neuromodulator to generate electrical stimulation pulses inaccordance with selected modulation parameters.

Implantable neuromodulation devices are active devices requiring energyfor operation, and thus, the neuromodulation system oftentimes includesan external charger to recharge a neuromodulation device, so that asurgical procedure to replace a power depleted neuromodulation devicecan be avoided. To wirelessly convey energy between the external chargerand the implanted neuromodulation device, the charger typically includesan alternating current (AC) charging coil that supplies energy to asimilar charging coil located in or on the neuromodulation device. Theenergy received by the charging coil located on the neuromodulationdevice can then be stored in a rechargeable battery within theneuromodulation device, which can then be used to power the electroniccomponentry on-demand. Depending on the settings, the neuromodulationdevice may need to be recharged every 1-30 days.

Electrical stimulation energy may be delivered from the neuromodulationdevice to the electrodes in the form of an electrical pulsed waveform.Thus, stimulation energy may be controllably delivered to the electrodesto stimulate neural tissue. The configuration of electrodes used todeliver electrical pulses to the targeted tissue constitutes anelectrode configuration, with the electrodes capable of beingselectively programmed to act as anodes (positive), cathodes (negative),or left off (zero). In other words, an electrode configurationrepresents the polarity being positive, negative, or zero. Otherparameters that may be controlled or varied include the amplitude, pulsewidth, and rate (or frequency) of the electrical pulses provided throughthe electrode array. Each electrode configuration, along with theelectrical pulse parameters, can be referred to as a “modulationparameter set.”

The lead or leads are typically placed in a location, such that theelectrical stimulation will cause paresthesia. The current understandingis that paresthesia induced by the stimulation and perceived by thepatient should be located in approximately the same place in thepatient's body as the pain that is the target of treatment. If a lead isnot correctly positioned, it is possible that the patient will receivelittle or no benefit from an implanted SCS system. Thus, correct leadplacement can mean the difference between effective and ineffective paintherapy. When electrical leads are implanted within the patient, thecomputerized programming system, in the context of an operating room(OR) mapping procedure, may be used to instruct the neuromodulationdevice to apply electrical stimulation to test placement of the leadsand/or electrodes, thereby assuring that the leads and/or electrodes areimplanted in effective locations within the patient.

Although alternative or artifactual sensations are usually appreciatedby patients, patients sometimes report these sensations to beuncomfortable, and therefore, they can be considered an adverseside-effect to neuromodulation therapy in some cases. It has been shownthat the delivery of sub-threshold electrical energy (e.g., high-ratepulsed electrical energy and/or low pulse width electrical energy) canbe effective in providing neuromodulation therapy for chronic painwithout causing paresthesia.

Once the leads are correctly positioned, a fitting procedure, which maybe referred to as a navigation session, may be performed using thecomputerized programming system to program the external control device,and if applicable the neuromodulation device, with a set of modulationparameters that best addresses the painful site. Thus, the navigationsession may be used to pinpoint the volume of activation (VOA) or areascorrelating to the pain. Such programming ability is particularlyadvantageous for targeting the tissue during implantation, or afterimplantation should the leads gradually or unexpectedly move that wouldotherwise relocate the stimulation energy away from the target site. Byreprogramming the neuromodulation device (typically by independentlyvarying the stimulation energy on the electrodes), the volume ofactivation (VOA) can often be moved back to the effective pain sitewithout having to re-operate on the patient in order to reposition thelead and its electrode array. When adjusting the volume of activation(VOA) relative to the tissue, it is desirable to make small changes inthe proportions of current, so that changes in the spatial recruitmentof nerve fibers will be perceived by the patient as being smooth andcontinuous and to have incremental targeting capability.

An external control device can be used to instruct the neuromodulationdevice to generate electrical stimulation pulses in accordance with theselected modulation parameters. Typically, the modulation parametersprogrammed into the neuromodulation device can be adjusted bymanipulating controls on the external control device to modify theelectrical stimulation provided by the neuromodulation device system tothe patient. Thus, in accordance with the modulation parametersprogrammed by the external control device, electrical pulses can bedelivered from the neuromodulation device to the stimulationelectrode(s) to stimulate or activate a volume of tissue in accordancewith a set of modulation parameters and provide the desired efficacioustherapy to the patient. The best modulation parameter set will typicallybe one that delivers stimulation energy to the volume of tissue thatmust be stimulated in order to provide the therapeutic benefit (e.g.,treatment of pain), while minimizing the volume of non-target tissuethat is stimulated.

The clinician generally programs the neuromodulation device through acomputerized programming system. This programming system can be aself-contained hardware/software system, or can be defined predominantlyby software running on a standard personal computer (PC). The PC orcustom hardware may actively control the characteristics of theelectrical stimulation generated by the neuromodulation device to allowthe optimum modulation parameters to be determined based on patientfeedback or other means and to subsequently program the neuromodulationdevice with the optimum modulation parameter set or sets. Thecomputerized programming system may be operated by a clinician attendingthe patient in several scenarios.

One known computerized programming system for SCS is called the BionicNavigator®, available from Boston Scientific NeuromodulationCorporation. The Bionic Navigator® is a software package that operateson a suitable PC and allows clinicians to program modulation parametersinto an external handheld programmer (referred to as a remote control).Each set of modulation parameters, including fractionalized currentdistribution to the electrodes (as percentage cathodic current,percentage anodic current, or off), may be stored in both the BionicNavigator® and the remote control and combined into a stimulationprogram that can then be used to stimulate multiple regions within thepatient.

A typical transverse section of the spinal cord will include a central“butterfly” shaped central area of gray matter (neuronal cell bodies)substantially surrounded by an ellipse-shaped outer area of white matter(myelinated axons). The dorsal horns are the dorsal portions of the“butterfly” shaped central area of gray matter, which includes neuronalcell terminals, neuronal cell bodies, dendrites, and axons. ConventionalSCS programming has as its therapeutic goal maximal stimulation (i.e.,recruitment) of dorsal column fibers that run in the white matter alongthe longitudinal axis of the spinal cord and minimal stimulation ofother fibers that run perpendicular to the longitudinal axis of thespinal cord (dorsal root fibers, predominantly). The white matter of thedorsal column includes mostly large myelinated axons that form afferentfibers.

While fibers in the dorsal column run in an axial direction, fibers inthe dorsal horn can be oriented in many directions, includingperpendicular to the longitudinal axis of the spinal cord. Dorsal hornfibers are also a different distance from the typically placed epiduralSCS leads, when compared to dorsal column fibers.

Further, dorsal horn fibers and dorsal column fibers have differentresponses to electrical stimulation. The strength of stimulation (i.e.,depolarizing or hyperpolarizing) of the dorsal column fibers and neuronsis described by the so-called “activating function” ∂V/∂x² which isproportional to the second-order spatial derivative of the voltage alongthe longitudinal axis of the spine. This is partially because the largemyelinated axons in dorsal column are primarily aligned longitudinallyalong the spine. On the other hand, the likelihood of generating actionpotentials in dorsal horn fibers and neurons is described by the“activating function” ∂V/∂x (otherwise known as the electric field). Thedorsal horn “activating function” is proportional not to thesecond-order derivative, but to the first-order derivative of thevoltage along the fiber axis. Accordingly, distance from the electricalfield locus affects the dorsal horn “activating function” less than itaffects the dorsal column “activating function.”

Current implantable neuromodulation systems typically include electrodesimplanted adjacent to the dorsal column of the spinal cord of thepatient. Current implantable neuromodulation systems are also typicallyprogrammed to deliver stimulation energy to the spinal withoutdifferentiating between the dorsal column and the dorsal horn of thespinal cord of the patient. While generally stimulation of neuronalelements (e.g., neurons, dendrites, axons, cell bodies, and neuronalcell terminals) in the patient's spinal cord provides therapy for pain,such stimulation sometimes causes alternative or artifactual sensations(e.g., paresthesia), which are sometime unwelcomed by the patient. Suchstimulation also requires (1) selective lead placement, as describedabove, (2) optimal stimulating electrode selection, and (3) optimizationof electrode configuration (e.g., polarity and anode-cathodeseparation). Accordingly, these exists a need for implantableneuromodulation systems and modulation parameter sets for same thatprovide therapy for pain while minimizing alternative or artifactualsensations and the sensitivity of the system to modulation parameters.There also exists a need for an implantable neuromodulation systems andmodulation parameter sets for same that preferentially stimulate dorsalhorn neuronal elements over dorsal column neuronal elements.

Current implantable neuromodulation methods also include an electricalfield localization step, in which the longitudinal location of theelectrical field locus is identified through trial and error and withpatient feedback. While electrical field localization provides effectiveneuromodulation, the process is time-consuming and requires patientparticipation. Further, when an electrical modulation lead shifts,repeating electrical field localization may be required. Accordingly,there exists a need for implantable neuromodulation systems andparameter sets for effective neuromodulation while minimizing the needfor electrical field localization.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate the design and utility of preferred embodimentsof the present invention, in which similar elements are referred to bycommon reference numerals. In order to better appreciate how theabove-recited and other advantages and objects of the present inventionsare obtained, a more particular description of the present inventionsbriefly described above will be rendered by reference to specificembodiments thereof, which are illustrated in the accompanying drawings.Understanding that these drawings depict only typical embodiments of theinvention and are not therefore to be considered limiting of its scope,the invention will be described and explained with additionalspecificity and detail through the use of the accompanying drawings inwhich:

FIG. 1 is a plan view of a Spinal Cord Modulation (SCM) systemconstructed in accordance with one embodiment of the present inventions;

FIG. 2 is a plan view of the SCM system of FIG. 1 in use with a patient;

FIG. 3 is a profile view of an implantable pulse generator (IPG) andpercutaneous leads used in the SCM system of FIG. 1;

FIG. 4 is a block diagram of the internal components of a clinician'sprogrammer (CP) used in the SCM system of FIG. 1;

FIG. 5 is a plan view of a user interface of the CP of FIG. 4 forprogramming the IPG of FIG. 3 in a manual programming mode;

FIG. 6 is a schematic view of an electrical modulation lead implantedadjacent a spinal cord in accordance with one embodiment of the presentinventions;

FIG. 7 is a schematic view of an electrical modulation lead inaccordance with one embodiment of the present inventions;

FIG. 8 is a plot of a longitudinal component of an electrical fieldsuperimposed over a schematic view of an electrical modulation lead inaccordance with one embodiment of the present inventions;

FIG. 9 is a plot of a transverse component of an electrical fieldsuperimposed over a schematic view of an electrical modulation leadimplanted adjacent a spinal cord in accordance with one embodiment ofthe present inventions;

FIG. 10 is a schematic view of two electrical modulation leads implantedadjacent a spinal cord in accordance with one embodiment of the presentinventions;

FIG. 11 is a schematic view of a myelinated axon in a dorsal column of aspinal cord.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present inventions, a method ofoperating an implantable neuromodulator coupled to an electrode arrayimplanted adjacent tissue of a patient having a medical condition isprovided. The method comprises conveying electrical modulation energy totissue of the patient in accordance with a modulation parameter set,wherein conveying the electrical modulation energy to tissue of thepatient in accordance with the modulation parameter set stimulatesdorsal horn neuronal elements more than dorsal column neuronal elements.

In one embodiment, conveying the electrical modulation energy to tissueof the patient in accordance with the modulation parameter set generatesan electrical field having a locus disposed adjacent a dorsal horn of aspinal cord of the patient. The locus may be disposed closer to thedorsal horn of the spinal cord of the patient than an ipsilateral dorsalcolumn of the spinal cord of the patient, wherein the dorsal column isadjacent the dorsal horn. The electrical modulation energy may beconveyed through electrodes on an implanted electrical modulation lead,and conveying the electrical modulation energy to tissue of the patientin accordance with the modulation parameter set may stimulate dorsalhorn neuronal elements along a substantial portion (i.e., more than 80%)of the implanted electrical modulation lead.

In another embodiment, conveying the electrical modulation energy totissue of the patient in accordance with the modulation parameter setgenerates an electrical field, wherein a first component of theelectrical field extends along a spinal cord of the patient, wherein asecond component of the electrical field extends transverse to thespinal cord of the patient, and wherein a first gradient across thefirst component of the electrical field renders the dorsal columnneuronal elements less excitable than a second gradient across thesecond component of the electrical field renders the dorsal hornneuronal elements.

In another embodiment, conveying the electrical modulation energy totissue of the patient in accordance with the modulation parameter setgenerates an electrical field, wherein a first component of theelectrical field extends along a spinal cord of the patient, wherein asecond component of the electrical field extends transverse to thespinal cord of the patient, and wherein a second gradient across thefirst component of the electrical field is weaker than a first gradientacross the second component of the electrical field.

In still another embodiment, conveying the electrical modulation energyto tissue of the patient in accordance with the modulation parameter setgenerates a first driving force in dorsal horn neuronal elements that isstronger than a second driving force in dorsal column neuronal elements.

In yet another embodiment, the modulation parameter set defines anelectrode combination. The electrode combination may comprise afractionalized electrode combination. The fractionalized electrodecombination may be configured such that all electrodes on an electricalmodulation lead have the same polarity. The fractionalized electrodecombination may be configured such that a plurality of electrodes on anelectrical modulation lead all have the same polarity, and no electrodeon the electrical modulation lead has the opposite polarity. Theplurality of electrodes may be disposed adjacent to each other on theelectrical modulation lead. The plurality of electrodes may have three,four, or five electrodes. The fractionalized electrode combination maybe configured such that all electrodes on the electrical modulation leadare anodes or cathodes. The fractionalized electrode combination may beconfigured such that an equal amount or unequal amounts of current isdirected to each electrode on the electrical modulation lead.

The fractionalized electrode combination may be further configured suchthat the same neuronal driving force is directed to each electrode ofthe plurality. The method may further comprise calculating a drivingforce directed to each electrode of the plurality by calibrating eachelectrode of the plurality.

In still another embodiment, conveying the electrical modulation energyto tissue of the patient in accordance with the modulation parameter setgenerates an electrical field having an elongated shape disposed over adorsal horn of a spinal cord of the patient, and the elongated shape hasa longitudinal axis parallel to the spinal cord of the patient. Theelongated shape may taper at the one or both ends thereof.

In accordance with a second aspect of the present inventions, a methodof operating an implantable neuromodulator coupled to an electrode arrayimplanted adjacent tissue of a patient having a medical condition isprovided. The method comprises conveying electrical modulation energy totissue of the patient in accordance with a modulation parameter set,wherein conveying the electrical modulation energy to tissue of thepatient in accordance with the modulation parameter set stimulatesdorsal horn neuronal elements more than dorsal column neuronal elements.The method also comprises determining a perception threshold forelectrical modulation energy conveyed through each electrode of aplurality of electrodes on an electrical modulation lead; modifying aneuronal driving force indicator for electrical modulation energyconveyed through each electrode of the plurality based on the respectivedetermined perception threshold. The method further comprises modifyingthe modulation parameter set based on the modified neuronal drivingforce indicator for electrical modulation energy conveyed through eachelectrode of the plurality before conveying electrical modulation energyto tissue of the patient in accordance with the modulation parameterset.

In one embodiment, the neuronal driving force indicator is an activatingfunction. The activating function may be a continuous activatingfunction determined by calculating the second-order spatial derivativeof the extracellular potential along an axon. The activating functionmay be a discrete activating function estimating by the formula:AF(n)=G_(a)/(π×d×l)×[V_(e)(n−1)−2 V_(e)(n)+V_(e)(n+1)], wherein G_(a) isthe axonal internodal conductance, d is the axonal diameter, l is thelength of the node of Ranvier, V_(e)(n) is the strength of the electricfield at the node for which the activating function is determined,V_(e)(n−1) is the strength of the electric field at the node precedingthe node for which the activating function is determined, and V_(e)(n+1)is the strength of the electric field at the node following the node forwhich the activating function is determined. The proportionalityconstants in the above formula may be eliminated from the equation andresult in identical optimization.

In another embodiment, the neuronal driving force indicator may be atotal driving function (“TDF”), which is a linear combination of theactivating function at multiple nodes. The TDF is described in U.S.Patent Application Ser. Nos. 61/427,027 and 61/427,059, both filed onDec. 23, 2010. Further details discussing the modeling of neuronalelements in response to an induced electric field are described in U.S.Pat. No. 7,627,384, Eduardo N. Warman, Modeling the Effects of ElectricFields on Nerve Fibers: Determination of Excitation Thresholds, IEEETransactions on Biomedical Engineering, Vol. 39, No. 12, December 1992,and U.S. Patent Application Ser. No. 61/427,059, filed Dec. 23, 2010.All of the references identified in this paragraph are expresslyincorporated herein by reference as though set forth in full.

In another embodiment, the method also comprises estimating the currentfractionalization by minimizing the integral of the square of thediscrete activating function over a longitudinal axis of the electricalmodulation lead while maximizing the electric field over the dorsal hornof the spinal cord of the patient, to thereby minimize the discreteactivating function of the dorsal column neuronal elements. The methodmay further comprises estimating the current fractionalization byminimizing the sum of the square of the discrete activating functiondivided by the determined perception threshold at each electrode on anelectrical modulation lead. The perception threshold may be determinedusing patient feedback. The perception threshold may also be determinedby measuring local field potentials (action potentials) at an electrodeof the plurality, or evoked potentials at distance (e.g., at the cortexin the brain). The electrode may be the same electrode through whichelectrical modulation energy is conveyed to determine the perceptionthreshold. Conveying the electrical modulation energy to tissue of thepatient in accordance with the modified modulation parameter set maygenerates a modified electrical field, wherein conveying the electricalmodulation energy to tissue of the patient in accordance with themodulation parameter set before modification generates a non-modifiedelectrical field, and wherein a first gradient across a component of themodified electrical field extending along a spinal cord of the patientis weaker than a second gradient across a component of the non-modifiedelectrical field extending along the spinal cord of the patient. Inother words, the modified electrical field (by taking the perceptionthreshold into account) generates a longitudinal field gradient that isless than the longitudinal field gradient generated by a non-modifiedelectrical field.

In accordance with a third aspect of the present inventions, a method ofoperating an implantable neuromodulator coupled to an electrode arrayimplanted adjacent tissue of a patient having a medical condition isprovided. The method comprises implanting a first electrical modulationlead adjacent a dorsal horn of a spinal cord of the patient. The methodalso comprises implanting a second electrical modulation lead adjacent adorsal column of the spinal cord of the patient. The method furthercomprises conveying electrical modulation energy through the first andsecond electrical modulation leads to tissue of the patient inaccordance with a modulation parameter set, wherein the electricalmodulation energy conveyed in accordance with the modulation parameterset stimulates dorsal horn neuronal elements more than dorsal columnneuronal elements.

In some embodiments, conveying the electrical modulation energy totissue of the patient in accordance with the modulation parameter setdoes not cause the patient to perceive paresthesia in response to theconveyed electrical modulation energy.

Other and further aspects and features of the invention will be evidentfrom reading the following detailed description of the preferredembodiments, which are intended to illustrate, not limit, the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Turning first to FIG. 1, an exemplary SCM system 10 generally includes aplurality (in this case, two) of implantable neuromodulation leads 12,an implantable pulse generator (IPG) 14, an external remote controllerRC 16, a clinician's programmer (CP) 18, an external trial modulator(ETM) 20, and an external charger 22.

The IPG 14 is physically connected via one or more percutaneous leadextensions 24 to the neuromodulation leads 12, which carry a pluralityof electrodes 26 arranged in an array. In the illustrated embodiment,the neuromodulation leads 12 are percutaneous leads, and to this end,the electrodes 26 are arranged in-line along the neuromodulation leads12. The number of neuromodulation leads 12 illustrated is two, althoughany suitable number of neuromodulation leads 12 can be provided,including only one, as long as the number of electrodes 26 is greaterthan two (including the IPG case) to allow for lateral steering of thecurrent. Alternatively, a surgical paddle lead can be used in place ofone or more of the percutaneous leads. As will be described in furtherdetail below, the IPG 14 includes pulse generation circuitry thatdelivers electrical modulation energy in the form of a pulsed electricalwaveform (i.e., a temporal series of electrical pulses) to the electrodearray 26 in accordance with a set of modulation parameters.

The ETM 20 may also be physically connected via the percutaneous leadextensions 28 and external cable 30 to the neuromodulation leads 12. TheETM 20, which has similar pulse generation circuitry as the IPG 14, alsodelivers electrical modulation energy in the form of a pulse electricalwaveform to the electrode array 26 accordance with a set of modulationparameters. The major difference between the ETM 20 and the IPG 14 isthat the ETM 20 is a non-implantable device that is used on a trialbasis after the neuromodulation leads 12 have been implanted and priorto implantation of the IPG 14, to test the responsiveness of themodulation that is to be provided. Thus, any functions described hereinwith respect to the IPG 14 can likewise be performed with respect to theETM 20. For purposes of brevity, the details of the ETM 20 will not bedescribed herein. Details of exemplary embodiments of ETM are disclosedin U.S. Pat. No. 6,895,280, which is expressly incorporated herein byreference.

The RC 16 may be used to telemetrically control the ETM 20 via abi-directional RF communications link 32. Once the IPG 14 andneuromodulation leads 12 are implanted, the RC 16 may be used totelemetrically control the IPG 14 via a bi-directional RF communicationslink 34. Such control allows the IPG 14 to be turned on or off and to beprogrammed with different modulation parameter sets. The IPG 14 may alsobe operated to modify the programmed modulation parameters to activelycontrol the characteristics of the electrical modulation energy outputby the IPG 14. As will be described in further detail below, the CP 18provides clinician detailed modulation parameters for programming theIPG 14 and ETM 20 in the operating room and in follow-up sessions.

The CP 18 may perform this function by indirectly communicating with theIPG 14 or ETM 20, through the RC 16, via an IR communications link 36.Alternatively, the CP 18 may directly communicate with the IPG 14 or ETM20 via an RF communications link (not shown). The clinician detailedmodulation parameters provided by the CP 18 are also used to program theRC 16, so that the modulation parameters can be subsequently modified byoperation of the RC 16 in a stand-alone mode (i.e., without theassistance of the CP 18).

The external charger 22 is a portable device used to transcutaneouslycharge the IPG 14 via an inductive link 38. Once the IPG 14 has beenprogrammed, and its power source has been charged by the externalcharger 22 or otherwise replenished, the IPG 14 may function asprogrammed without the RC 16 or CP 18 being present. For purposes ofbrevity, the details of the external charger 22 will not be describedherein. Details of exemplary embodiments of the external charger aredisclosed in U.S. Pat. No. 6,895,280, which is expressly incorporatedherein by reference.

As shown in FIG. 2, the neuromodulation leads 12 are implanted withinthe spinal column 42 of a patient 40. The preferred placement of theneuromodulation leads 12 is adjacent, i.e., resting upon, the spinalcord area to be stimulated. Due to the lack of space near the locationwhere the neuromodulation leads 12 exit the spinal column 42, the IPG 14is generally implanted in a surgically-made pocket either in the abdomenor above the buttocks. The IPG 14 may, of course, also be implanted inother locations of the patient's body. The lead extension 24 facilitateslocating the IPG 14 away from the exit point of the neuromodulationleads 12. As shown in FIG. 2, the CP 18 communicates with the IPG 14 viathe RC 16.

Referring now to FIG. 3, the external features of the neuromodulationleads 12 and the IPG 14 will be briefly described. One of theneuromodulation leads 12 a has eight electrodes 26 (labeled E1-E8), andthe other neuromodulation lead 12 b has eight electrodes 26 (labeledE9-E16). The actual number and shape of leads and electrodes will, ofcourse, vary according to the intended application. The IPG 14 comprisesan outer case 44 for housing the electronic and other components(described in further detail below), and a connector 46 to which theproximal ends of the neuromodulation leads 12 mates in a manner thatelectrically couples the electrodes 26 to the electronics within theouter case 44. The outer case 44 is composed of an electricallyconductive, biocompatible material, such as titanium, and forms ahermetically sealed compartment wherein the internal electronics areprotected from the body tissue and fluids. In some cases, the outer case44 may serve as an electrode.

The IPG 14 comprises electronic components, such as acontroller/processor (e.g., a microcontroller) 39, memory 41, a battery43, telemetry circuitry 45, monitoring circuitry 47, modulation outputcircuitry 49, and other suitable components known to those skilled inthe art. The microcontroller 39 executes a suitable program stored inmemory 41, for directing and controlling the neuromodulation performedby IPG 14. Telemetry circuitry 45, including an antenna (not shown), isconfigured for receiving programming data (e.g., the operating programand/or modulation parameters) from the RC 16 and/or CP 18 in anappropriate modulated carrier signal, which the programming data is thenstored in the memory (not shown). The telemetry circuitry 45 is alsoconfigured for transmitting status data to the RC 16 and/or CP 18 in anappropriate modulated carrier signal. The battery 43, which may be arechargeable lithium-ion or lithium-ion polymer battery, providesoperating power to IPG 14. The monitoring circuitry 47 is configured formonitoring the present capacity level of the battery 43.

The modulation output circuitry 49 provides electrical modulation energyin the form of a pulsed electrical waveform to the electrodes 26 inaccordance with a set of modulation parameters programmed into the IPG14. Such modulation parameters may comprise electrode combinations,which define the electrodes that are activated as anodes (positive),cathodes (negative), and turned off (zero), percentage of modulationenergy assigned to each electrode (fractionalized electrodeconfigurations), and electrical pulse parameters, which define the pulseamplitude (measured in milliamps or volts depending on whether the IPG14 supplies constant current or constant voltage to the electrode array26), pulse width (measured in microseconds), pulse rate (measured inpulses per second), and burst rate (measured as the modulation onduration X and modulation off duration Y).

Electrical modulation will occur between a plurality of activatedelectrodes, one of which may be the IPG case 44. The system 10 iscapable of transmitting modulation energy to the tissue in a monopolaror multipolar (e.g., bipolar, tripolar, etc.) fashion, but monopolarmodulation is used in the disclosed method. Monopolar modulation occurswhen a selected one of the lead electrodes 26 is activated along withthe case of the IPG 14, so that modulation energy is transmitted betweenthe selected electrode 26 and case.

Any of the electrodes E1-E16 and case electrode may be assigned to up tok possible groups or timing “channels.” In one embodiment, k may equalfour. The timing channel identifies which electrodes are selected tosynchronously source or sink current to create an electric field in thetissue to be stimulated. Amplitudes and polarities of electrodes on achannel may vary. In particular, the electrodes can be selected to bepositive (anode, sourcing current), negative (cathode, sinking current),or off (no current) polarity in any of the k timing channels.

In the illustrated embodiment, IPG 14 can individually control themagnitude of electrical current flowing through each of the electrodes.In this case, it is preferred to have a current generator, whereinindividual current-regulated amplitudes from independent current sourcesfor each electrode may be selectively generated. Although this system isoptimal to take advantage of the invention, other neuromodulators thatmay be used with the invention include neuromodulators having voltageregulated outputs. While individually programmable electrode amplitudesare optimal to achieve fine control, a single output source switchedacross electrodes may also be used, although with less fine control inprogramming. Mixed current and voltage regulated devices may also beused with the invention. Further details discussing the detailedstructure and function of IPGs are described more fully in U.S. Pat.Nos. 6,516,227 and 6,993,384, which are expressly incorporated herein byreference.

It should be noted that rather than an IPG, the SCM system 10 mayalternatively utilize an implantable receiver-stimulator (not shown)connected to the neuromodulation leads 12. In this case, the powersource, e.g., a battery, for powering the implanted receiver, as well ascontrol circuitry to command the receiver-stimulator, will be containedin an external controller inductively coupled to the receiver-stimulatorvia an electromagnetic link. Data/power signals are transcutaneouslycoupled from a cable-connected transmission coil placed over theimplanted receiver-modulator. The implanted receiver-modulator receivesthe signal and generates the modulation in accordance with the controlsignals.

The IPG 14 may be operated in one of a super-threshold delivery mode anda sub-threshold delivery mode. While in the super-threshold deliverymode, the IPG 14 is configured for delivering electrical modulationenergy that provides super-threshold therapy to the patient (in thiscase, causes the patient to perceive paresthesia). While in thesub-threshold delivery mode, the IPG 14 is configured for deliveringelectrical modulation energy that provides sub-threshold therapy to thepatient (in this case, does not cause the patient to perceiveparesthesia). Further details discussing modulation phases and deliverymodes are described more fully in U.S. Provisional Patent ApplicationSer. No. 61/801,917, entitled “Systems and Methods for DeliveringSub-Threshold Therapy to a Patient,” which is expressly incorporatedherein by reference.

As briefly discussed above, the CP 18 greatly simplifies the programmingof multiple electrode configurations, allowing the user (e.g., thephysician or clinician) to readily determine the desired modulationparameters to be programmed into the IPG 14, as well as the RC 16. Thus,modification of the modulation parameters in the programmable memory ofthe IPG 14 after implantation is performed by a user using the CP 18,which can directly communicate with the IPG 14 or indirectly communicatewith the IPG 14 via the RC 16. That is, the CP 18 can be used by theuser to modify operating parameters of the electrode array 26 near thespinal cord.

As shown in FIG. 2, the overall appearance of the CP 18 is that of alaptop personal computer (PC), and in fact, may be implemented using aPC that has been appropriately configured to include adirectional-programming device and programmed to perform the functionsdescribed herein. Alternatively, the CP 18 may take the form of amini-computer, personal digital assistant (PDA), etc., or even a remotecontrol (RC) with expanded functionality. Thus, the programmingmethodologies can be performed by executing software instructionscontained within the CP 18. Alternatively, such programmingmethodologies can be performed using firmware or hardware. In any event,the CP 18 may actively control the characteristics of the electricalstimulation generated by the IPG 14 to allow the optimum modulationparameters to be determined based on patient feedback and forsubsequently programming the IPG 14 with the optimum modulationparameter.

To allow the user to perform these functions, the CP 18 includes a userinput device (e.g., a mouse 72 and a keyboard 74), and a programmingdisplay screen 76 housed in a case 78. It is to be understood that inaddition to, or in lieu of, the mouse 72, other directional programmingdevices may be used, such as a trackball, touchpad, joystick, ordirectional keys included as part of the keys associated with thekeyboard 74.

In the illustrated embodiment described below, the display screen 76takes the form of a conventional screen, in which case, a virtualpointing device, such as a cursor controlled by a mouse, joy stick,trackball, etc., can be used to manipulate graphical objects on thedisplay screen 76. In alternative embodiments, the display screen 76takes the form of a digitizer touch screen, which may either passive oractive. If passive, the display screen 76 includes detection circuitry(not shown) that recognizes pressure or a change in an electricalcurrent when a passive device, such as a finger or non-electronicstylus, contacts the screen. If active, the display screen 76 includesdetection circuitry that recognizes a signal transmitted by anelectronic pen or stylus. In either case, detection circuitry is capableof detecting when a physical pointing device (e.g., a finger, anon-electronic stylus, or an electronic stylus) is in close proximity tothe screen, whether it be making physical contact between the pointingdevice and the screen or bringing the pointing device in proximity tothe screen within a predetermined distance, as well as detecting thelocation of the screen in which the physical pointing device is in closeproximity. When the pointing device touches or otherwise is in closeproximity to the screen, the graphical object on the screen adjacent tothe touch point is “locked” for manipulation, and when the pointingdevice is moved away from the screen the previously locked object isunlocked. Further details discussing the use of a digitizer screen forprogramming are set forth in U.S. Provisional Patent Application Ser.No. 61/561,760, entitled “Technique for Linking Electrodes Togetherduring Programming of Neurostimulation System,” which is expresslyincorporated herein by reference.

As shown in FIG. 4, the CP 18 includes a controller/processor 80 (e.g.,a central processor unit (CPU)) and memory 82 that stores a stimulationprogramming package 84, which can be executed by thecontroller/processor 80 to allow the user to program the IPG 14, and RC16. The CP 18 further includes an output circuitry 86 for downloadingmodulation parameters to the IPG 14 and RC 16 and for uploadingmodulation parameters already stored in the memory 66 of the RC 16 ormemory of the IPG 14. In addition, the CP 18 further includes a userinput device 88 (such as the mouse 72 or keyboard 74) to provide usercommands. Notably, while the controller/processor 80 is shown in FIG. 4as a single device, the processing functions and controlling functionscan be performed by a separate controller and processor 64. Thus, it canbe appreciated that the controlling functions described below as beingperformed by the CP 18 can be performed by a controller, and theprocessing functions described below as being performed by the CP 18 canbe performed by the processor 64.

Execution of the programming package 84 by the controller/processor 80provides a multitude of display screens (not shown) that can benavigated through via use of the mouse 72. These display screens allowthe clinician to, among other functions, to select or enter patientprofile information (e.g., name, birth date, patient identification,physician, diagnosis, and address), enter procedure information (e.g.,programming/follow-up, implant trial system, implant IPG, implant IPGand lead(s), replace IPG, replace IPG and leads, replace or reviseleads, explant, etc.), generate a pain map of the patient, define theconfiguration and orientation of the leads, initiate and control theelectrical modulation energy output by the neuromodulation leads 12, andselect and program the IPG 14 with modulation parameters in both asurgical setting and a clinical setting. Further details discussing theabove-described CP functions are disclosed in U.S. patent applicationSer. No. 12/501,282, entitled “System and Method for Converting TissueStimulation Programs in a Format Usable by an Electrical CurrentSteering Navigator,” and U.S. patent application Ser. No. 12/614,942,entitled “System and Method for Determining Appropriate Steering Tablesfor Distributing Modulation energy Among Multiple NeuromodulationElectrodes,” which are expressly incorporated herein by reference.Execution of the programming package 84 provides a user interface thatconveniently allows a user to program the IPG 14.

Referring to FIG. 5, a graphical user interface (GUI) 100 that can begenerated by the CP 18 to allow a user to program the IPG 14 will bedescribed. In the illustrated embodiment, the GUI 100 comprises threepanels: a program selection panel 102, a lead display panel 104, and amodulation parameter adjustment panel 106. Some embodiments of the GUI100 may allow for closing and expanding one or both of the lead displaypanel 102 and the parameter adjustment panel 106 by clicking on the tab108 (to show or hide the parameter adjustment panel 106) or the tab 110(to show or hide the full view of both the lead selection panel 104 andthe parameter adjustment panel 106).

The program selection panel 102 provides information about modulationprograms and coverage areas that have been, or may be, defined for theIPG 14. In particular, the program selection panel 102 includes acarousel 112 on which a plurality of modulation programs 114 (in thiscase, up to sixteen) may be displayed and selected. The programselection panel 102 further includes a selected program status field 116indicating the number of the modulation program 114 that is currentlyselected (any number from “1” to “16”). In the illustrated embodiment,program 1 is the only one currently selected, as indicated by the number“1” in the field 116. The program selection panel 102 further comprisesa name field 118 in which a user may associate a unique name to thecurrently selected modulation program 114. In the illustratedembodiment, currently selected program 1 has been called “lower back,”thereby identifying program 1 as being the modulation program 114designed to provide therapy for lower back pain.

The program selection panel 102 further comprises a plurality ofcoverage areas 120 (in this case, up to four) with which a plurality ofmodulation parameter sets can respectively be associated to create thecurrently selected modulation program 114 (in this case, program 1).Each coverage area 120 that has been defined includes a designationfield 122 (one of letters “A”-“D”), and an electrical pulse parameterfield 124 displaying the electrical pulse parameters, and specifically,the pulse amplitude, pulse width, and pulse rate, of the modulationparameter set associated with the that coverage area. In this example,only coverage area A is defined for program 1, as indicated by the “A”in the designation field 122. The electrical pulse parameter field 124indicates that a pulse amplitude of 5 mA, a pulse width of 210 μs, and apulse rate of 40 Hz has been associated with coverage area A.

Each of the defined coverage areas 120 also includes a selection icon126 that can be alternately actuated to activate or deactivate therespective coverage area 120. When a coverage area is activated, anelectrical pulse train is delivered from the IPG 14 to the electrodearray 26 in accordance with the modulation parameter set associated withthat coverage area. Notably, multiple ones of the coverage areas 120 canbe simultaneously activated by actuating the selection icons 126 for therespective coverage areas. In this case, multiple electrical pulsetrains are concurrently delivered from the IPG 14 to the electrode array26 during timing channels in an interleaved fashion in accordance withthe respective modulation parameter sets associated with the coverageareas 120. Thus, each coverage area 120 corresponds to a timing channel.

To the extent that any of the coverage areas 120 have not been defined(in this case, three have not been defined), they include text “click toadd another program area”), indicating that any of these remainingcoverage areas 120 can be selected for association with a modulationparameter set. Once selected, the coverage area 120 will be populatedwith the designation field 122, electrical pulse parameter field 124,and selection icon 126.

The lead display panel 104 includes graphical leads 128, which areillustrated with eight graphical electrodes 130 each (labeled electrodesE1-E8 for the first lead 128 and electrodes E9-E16 for second lead 128,see FIG. 10 for corresponding modulation leads 12). The lead displaypanel 104 also includes a graphical case 132 representing the case 44 ofthe IPG 14. The lead display panel 104 further includes lead groupselection tabs 134 (in this case, four), any of which can be actuated toselect one of four groups of graphical leads 128. In this case, thefirst lead group selection tab 134 has been actuated, thereby displayingthe two graphical leads 128 in their defined orientation. In the casewhere additional leads 12 are implanted within the patient, they can beassociated with additional lead groups.

The parameters adjustment panel 106 also includes a pulse amplitudeadjustment control 136 (expressed in milliamperes (mA)), a pulse widthadjustment control 138 (expressed in microseconds (μs)), and a pulserate adjustment control 140 (expressed in Hertz (Hz)), which aredisplayed and actuatable in all the programming modes. Each of thecontrols 136-140 includes a first arrow that can be actuated to decreasethe value of the respective modulation parameter and a second arrow thatcan be actuated to increase the value of the respective modulationparameter. Each of the controls 136-140 also includes a display area fordisplaying the currently selected parameter. In response to theadjustment of any of electrical pulse parameters via manipulation of thegraphical controls in the parameter adjustment panel 106, thecontroller/processor 80 generates a corresponding modulation parameterset (with a new pulse amplitude, new pulse width, or new pulse rate) andtransmits it to the IPG 14 via the telemetry circuitry 86 for use indelivering the modulation energy to the electrodes 26.

The parameter adjustment panel 106 includes a pull-down programming modefield 142 that allows the user to switch between several programmingmodes, including a manual programming mode. Each of these programmingmodes allows a user to define a modulation parameter set for thecurrently selected coverage area 120 of the currently selected program114 via manipulation of graphical controls in the parameter adjustmentpanel 106 described above, as well as the various graphical controlsdescribed below. In the illustrated embodiment, when switching betweenprogramming modes via actuation of the programming mode field 142, thelast electrode configuration with which the IPG 14 was programmed in theprevious programming mode is converted into another electrodeconfiguration, which is used as the first electrode configuration withwhich the IPG 14 is programmed in the subsequent programming mode.

Using the CP 18, a user (e.g., a physician) instructs the IPG 14 togenerate a modulation signal resulting in an electrical field having alocus. The user can use either the manual programming mode (described inexamples below) of the CP 18 to set various modulation parameters. Theelectrical field locus can be displaced by fractionalizing the cathodiccurrent across the array of electrodes 26. Other modulation parameters(e.g., amplitude, frequency, duty cycle, pulse width, etc.) can also beadjusted.

Having described the hardware and software of the SCM system 10,placement of the electrical modulation lead 12 and the correspondingelectrical fields will now be described. FIGS. 6-9 illustrate thedifference in electrical field strength in the longitudinal andtransverse directions when the current is fractionalized such that theelectrical field in the longitudinal direction generated by thefractionalized current delivered to each electrode 26 is approximatelyequal. The voltage at a patient's spinal cord 11 (especially at thedorsal column fibers) is approximately equal in the longitudinaldirection, resulting in a voltage gradient of approximately zero alongthe dorsal column. However that may correspond to different amounts offractionalized current delivered to each electrode 26. Calibrationtechniques (also described below) are used to determine the propercurrent fractionalization. With the current fractionalized to aplurality of electrodes 26 on the electrical modulation lead 12, theresulting field can be calculated by superimposing the fields generatedby the current delivered to each electrode 26. Moreover each electricalfield has a longitudinal component and a transverse component.

FIG. 6 is a schematic view of a single electrical modulation lead 12implanted over approximately the longitudinal midline of the patient'sspinal cord 11. Longitudinal component of the electrical field isdirected along the y-axis depicted in FIG. 6, and a transverse componentof the electrical field is directed along the x-axis depicted in FIG. 6.

FIG. 7 is a schematic view of the electrical modulation lead 12 showingthe fractionalization of the anodic current delivered to the electrodes26 on the electrical modulation lead 12. As in FIG. 5, the outer case 44of the IPG 14 is the only cathode, and carries 100% of the cathodiccurrent. The fractionalization of the anodic current shown in FIG. 7does not deliver an equal amount of current to each electrode 26,because this embodiment takes into account the differences in how thetissue underlying each electrode 26 reacts to electrical stimulation(described below). Also, the ends of the portion of the electricalmodulation lead 12 at include electrodes 26 having lower gradient in thelongitudinal direction. The magnitude of the electrical field tapersdown at the ends of the electrical modulation lead 12. Fractionalizationof the current to the electrodes 26 is controlled such that the tissueunderlying each electrode 26 in the middle portion of the electricalmodulation lead 12 reacts approximately equally to the electricalstimulation, or tissue activation underlying each electrode areeliminated. However, the resulting fractionalization is not equal. Inthe embodiment shown in FIG. 7, fractionalization of the current to themiddle electrodes 26 varies from 10% to 18%, reflecting the variation inthe tissue underlying those electrodes 26. The fractionalization acrossthe electrical modulation lead 12 can vary in any manner as long as thetotal of fractionalized currents equals 100%.

The gradient in the longitudinal direction along the axis of theelectrical modulation lead 12 is schematically illustrated in FIG. 8. InFIG. 8, the electrical field strength 15 in the longitudinal directionis plotted over a schematic representation of the electrical modulationlead 12. FIG. 8 shows that the electrical field strength 15 issubstantially constant over the middle portion of the electricalmodulation lead 12, but may form a wave with very small amplitudebecause of the gaps between the electrodes 26 in the lead 12. Thissubstantially constant electrical field forms a small longitudinalgradient, which minimizes activation of the large myelinated axons inthe dorsal column. FIG. 8 also shows the electrical field in thelongitudinal direction tapering at the ends of the electrical modulationlead 12.

The gradient in the transverse direction is schematically illustrated inFIG. 9. In FIG. 9, the transverse electrical field strength 17 in thetransverse direction is plotted over a schematic representation of theelectrical modulation lead 12 and the spinal cord 11 of the patient.FIG. 9 shows that the transverse electrical field strength 17 isgreatest adjacent the electrical modulation lead 12 and falls offlateral of the electrical modulation lead 12. This field strength 17drop-off forms a sizable gradient in the transverse direction, whichactivates the neural cell terminals in the dorsal horn. The generatedelectrical field has an elongate, or stripe, shape with a tapering ateither or both ends of the stripe to minimize stimulation of the dorsalcolumn.

The stripe shape of the generated electrical field stimulates the dorsalhorn across a large longitudinal span (i.e., along most or all of thelead 12), thereby eliminating the need for electrical field localizationin neuromodulation. A subsequent step or series of steps could be usedto reduce the span of dorsal horn stimulation to save energy. Also, thesubstantially constant longitudinal electrical field and the largegradient in the transverse electrical field favor stimulation of dorsalhorn neuronal elements over dorsal column neuronal elements. Thiselectrical field makes the dorsal column neuronal elements even lessexcitable relative to the dorsal horn neuronal elements.

FIG. 10 depicts two electrical modulation leads 12 have been implantedover the spinal cord 11 of the patient. One of the electrical modulationlead 12 has been implanted more laterally with respect to the spinalcord 11, thereby placing it proximate the dorsal horn 13 of the spinalcord 11. The other electrical modulation lead 12 has been implanted moremedially with respect to the spinal cord 11, thereby placing itproximate the dorsal column of the spinal cord 11. Use of the SCM systemto fractionalize the current to the electrical modulation leads 12 andthe case 44 of the IPG 14 is depicted in FIG. 5. While FIG. 10 depictstwo electrical modulation leads 12, any other plurality of leads 12 or amultiple column paddle lead can also be used with the disclosed method.

As shown FIG. 5, the manual programming mode has been selected. In themanual programming mode, each of the electrodes 130 of the graphicalleads 128, as well as the graphical case 132, may be individuallyselected, allowing the clinician to set the polarity (cathode or anode)and the magnitude of the current (percentage) allocated to thatelectrode 130, 132 using graphical controls located in anamplitude/polarity area 144 of the parameter adjustment panel 106.Electrode E15 is shown as being selected to allow the user tosubsequently allocate the polarity and fractionalized electrical currentto it via the graphical controls located in the amplitude/polarity area144.

In particular, a graphical polarity control 146 located in theamplitude/polarity area 144 includes a “+” icon, a “−” icon, and an“OFF” icon, which can be respectively actuated to toggle the selectedelectrode 130, 132 between a positive polarization (anode), a negativepolarization (cathode), and an off-state. An amplitude control 148 inthe amplitude/polarity area 144 includes an arrow that can be actuatedto decrease the magnitude of the fractionalized current of the selectedelectrode 130, 132, and an arrow that can be actuated to increase themagnitude of the fractionalized current of the selected electrode 130,132. The amplitude control 148 also includes a display area thatindicates the adjusted magnitude of the fractionalized current for theselected electrode 134. The amplitude control 148 is preferably disabledif no electrode is visible and selected in the lead display panel 104.In response to the adjustment of fractionalized electrode combinationvia manipulation of the graphical controls in the amplitude/polarityarea 144, the controller/processor 80 generates a correspondingmodulation parameter set (with a new fractionalized electrodecombination) and transmits it to the IPG 14 via the telemetry circuitry86 for use in delivering the modulation energy to the electrodes 26.

In the embodiment illustrated in FIG. 5, the graphical case 132representing the case 44 of the IPG 14 has been selected as the cathodeto which 100% of the cathodic current has been allocated, and electrodesE1 to E8 have each been selected as an anode to which 12.5% of theanodic current has been allocated. Accordingly, 100% of the anodiccurrent has been distributed evenly across electrodes E1 to E8.Electrodes E1 to E8 correspond to electrodes 26 on the electricalmodulation lead 12 implanted adjacent the dorsal horn 13. Thisembodiment approximates an electrical field with a small longitudinalgradient by making the broad assumption that the tissue over which eachof electrodes E1 to E8 response similarly to electrical modulationenergy delivered through the respective electrodes. Under this broadassumption, the electrical field generated by the electrical modulationenergy delivered to the electrodes 130, 132 has a very small gradient inthe longitudinal direction along the axis of the electrical modulationlead 12 represented by the graphical lead 128 on which electrodes E1 toE8 are displayed. The small gradient in the longitudinal directionavoids activation of the myelinated axons in the dorsal column. On theother hand, the electrical field has a sizable gradient in thetransverse direction, which can activate the neural cell terminals inthe dorsal horn.

Although the graphical controls located in the amplitude/polarity area144 can be manipulated for any of the electrodes, a dedicated graphicalcontrol for selecting the polarity and fractionalized current value canbe associated with each of the electrodes, as described in U.S. PatentPublication No. 2012/0290041, entitled “Neurostimulation System withOn-Effector Programmer Control,” which is expressly incorporated hereinby reference.

The parameters adjustment panel 106, when the manual programming mode isselected, also includes an equalization control 150 that can be actuatedto automatically equalize current allocation to all electrodes of apolarity selected by respective “Anode +” and “Cathode −” icons. Theranges of pulse rates and pulse widths of the modulation parameter setsdefined during the manual programming can result in eithersuper-threshold therapy and sub-threshold therapy. For example, thelower limit of the pulse amplitude may be as low as 0.1 mA, wherein asthe upper limit of the pulse amplitude may be as high as 20 mA. Thelower limit of the pulse width may be as low as 2 μs, whereas the upperlimit of the pulse width may be as high as 1000 μs. For example, thelower limit of the pulse rate may be as low as 1 Hz, whereas the upperlimit of the pulse rate may be as high as 50 KHz. In the illustratedembodiment, a pulse amplitude of 5 mA, a pulse width of 210 μs, and apulse rate of 40 Hz have been selected. Thus, during the manualprogramming mode, the selected coverage area 120 of the selected program114 can be programmed with a modulation parameter set designed to eitherdeliver super-threshold therapy or sub-threshold therapy to the patient.

Another embodiment of a method of programming an SCM system 10 includesmodifying the fractionalized current delivered to each electrode 26 tominimize the electrical field gradient in the longitudinal direction, soas to minimize activation of the dorsal column neuronal elements.Minimizing activation of the dorsal or neuronal elements can include amodel-based calculation, where the model includes the information fromthe manual or auto-calibration (described below). As shown in FIG. 11,the myelinated large axons 19 in the dorsal column have gaps in themyelination 21 that formed nodes 23. The anatomy depicted in FIG. 11implies that the driving force for the myelinated large axons 19generated by current delivered to an electrode 26 can be approximatedwith an activating function.

The discrete activating function can be calculated by the formula:AF(n)=G_(a)/(π×d×l)×[V_(e)(n−1)−2 V_(e)(n)+V_(e)(n+1)], wherein G_(a) isthe axonal intermodal conductance, d is the axonal diameter, l is thelength of the node of Ranvier, V_(e)(n) is the strength of the electricfield at the node for which the activating function is determined,V_(e)(n−1) is the strength of the electric field at the node precedingthe node 23 for which the activating function is determined, andV_(e)(n+1) is the strength of the electric field at the node 23following the node 23 for which the activating function is determined.Using this formula, the discrete activating function is calculated fromthe conductance normalize to the surface area of the node of Ranvier.

The perception threshold is the modulation signal level above which apatient feels paresthesia. Because the perception threshold varies frompatient to patient and from electrode 26 to electrode 26 within apatient, a more accurate fractionalization of the current betweenelectrodes 26 requires modification of the fractionalization based onthe perception threshold at each electrode. The perception threshold canbe determined using the SCM system as described below.

A user (e.g., patient or clinician) can place the IPG 14 into PerceptionThreshold Identification Mode using the RC 16 or the CP 18,respectively. Once Perception Threshold Identification Mode isinitiated, the IPG 14 is directed to sequentially deliver modulationenergy to each of the electrodes 26 on a lead 12 at incrementallyincreasing modulation signals.

The IPG 14 may be configured for automatically incrementally increasingthe modulation signal parameters of the electrical pulse train withoutfurther user intervention or may be configured for incrementallyincreasing the modulation signal parameters of the electrical pulsetrain delivered by the IPG 14 each time the user actuates a controlelement on the RC 16 or the CP 18.

The RC 16 or the CP 18 is configured for prompting the patient toactuate a control element, once paresthesia is perceived by the patient.In response to this user input, the RC 16 or the CP is configured tostore the modulation signal strength of the electrical pulse traindelivered when the control element is actuated. This modulation signalstrength is identified as the perception threshold for the particularelectrode 26.

Alternatively, rather than relying on voluntary user input, the RC 16 orthe CP 18 may be configured for automatically identifying the perceptionthreshold in response to a sensed physiological parameter indicative ofsuper-threshold stimulation of the neural tissue (e.g., actionpotentials sensed by the IPG 14 at one or more electrodes 26 as a resultof the delivery of the modulation energy (“local field potentials”)).The above-described method for identifying a perception threshold may berepeated to identify the perception threshold at each of the electrodes26 on a lead 12.

The identified perception thresholds can be used to estimate the currentfractionalization by minimizing the sum of the square of the discreteactivating function divided by the determined perception threshold ateach electrode 26 on an electrical modulation lead 12. Squaring thediscrete activating function, or any driving force from the electricalfield, eliminates the differences in depolarizing and hyperpolarizingfields. The current fractionalization that results in a minimize summinimizes the field gradient in the longitudinal direction.

Although particular embodiments of the present inventions have beenshown and described, it will be understood that it is not intended tolimit the present inventions to the preferred embodiments, and it willbe obvious to those skilled in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe present inventions. Thus, the present inventions are intended tocover alternatives, modifications, and equivalents, which may beincluded within the spirit and scope of the present inventions asdefined by the claims.

What is claimed is:
 1. A non-transitory machine-readable medium including instructions, which when executed by a machine, cause the machine to deliver an electrical waveform from an implantable neuromodulator to at least some electrodes in an electrode array in accordance with a modulation parameter set to generate an electric field to stimulate dorsal horn neuronal elements more than dorsal column neuronal elements, wherein: a first component of the electrical field generated using the electrical waveform extends along a spinal cord of the patient and a second component of the electrical field generated using the electrical waveform extends transverse to the spinal cord of the patient; and a gradient across the first component of the electrical field renders the dorsal column neuronal elements less excitable than the dorsal horn neuronal elements are rendered by a gradient across the second component of the electrical field.
 2. The non-transitory machine-readable medium of claim 1, wherein the electrical waveform delivered in accordance with the modulation parameter set parameter set is configured to generate a first driving force in dorsal horn neuronal elements that is stronger than a second driving force in dorsal column neuronal elements.
 3. The non-transitory machine-readable medium of claim 1, wherein the modulation parameter set defines an electrode combination, wherein the electrode combination comprises a fractionalized electrode combination.
 4. The non-transitory machine-readable medium of claim 3, wherein the fractionalized electrode combination is configured such that all electrodes on an electrical modulation lead have the same polarity.
 5. The non-transitory machine-readable medium of claim 1, wherein the electrical field has an elongated shape disposed over a dorsal horn of a spinal cord of the patient, and the elongated shape has a longitudinal axis parallel to the spinal cord of the patient.
 6. The non-transitory machine-readable medium of claim 1, wherein the generated electric field does not cause the patient to perceive paresthesia.
 7. The non-transitory machine-readable medium of claim 1, wherein the instructions, which when executed by a machine, cause the machine to: determine a perception threshold for electrical modulation energy conveyed through each electrode of a plurality of electrodes on an electrical modulation lead; modify a neuronal driving force indicator for electrical modulation energy conveyed through each electrode of the plurality based on the respective determined perception threshold; and modify the modulation parameter set based on the modified neuronal driving force indicator for electrical modulation energy conveyed through each electrode of the plurality before delivering the electrical waveform in accordance with the modulation parameter set.
 8. The non-transitory machine-readable medium of claim 7, wherein the neuronal driving force indicator is an activating function.
 9. The non-transitory machine-readable medium of claim 7, wherein the perception threshold is determined using patient feedback.
 10. The non-transitory machine-readable medium of claim 7, wherein the perception threshold is determined by measuring local field potentials.
 11. A system for use with an implantable electrode array, the system comprising: an implantable neuromodulator configured to be coupled to the implantable electrode array, the implantable neuromodulator including a modulation output circuitry to generate an electrical waveform and a control circuitry configured to cooperate with the modulation output circuitry to deliver the electrical waveform to at least some electrodes in the electrode array in accordance with a modulation parameter set to generate an electric field to stimulate dorsal horn neuronal elements more than dorsal column neuronal element, wherein: a first component of the electrical field generated using the electrical waveform extends along a spinal cord of the patient and a second component of the electrical field generated using the electrical waveform extends transverse to the spinal cord of the patient; and a gradient across the first component of the electrical field renders the dorsal column neuronal elements less excitable than the dorsal horn neuronal elements are rendered by a gradient across the second component of the electrical field.
 12. The system of claim 11, wherein the electrical waveform delivered in accordance with the modulation parameter set parameter set is configured to generate a first driving force in dorsal horn neuronal elements that is stronger than a second driving force in dorsal column neuronal elements.
 13. The system of claim 11, wherein the modulation parameter set defines an electrode combination, wherein the electrode combination comprises a fractionalized electrode combination.
 14. The system of claim 13, wherein the fractionalized electrode combination is configured such that all electrodes on an electrical modulation lead have the same polarity.
 15. The system of claim 11, wherein the electrical field has an elongated shape disposed over a dorsal horn of a spinal cord of the patient, and the elongated shape has a longitudinal axis parallel to the spinal cord of the patient.
 16. The system of claim 11, wherein the control circuitry configured to cooperate with the modulation output circuitry to: determine a perception threshold for electrical modulation energy conveyed through each electrode of a plurality of electrodes on an electrical modulation lead; modify a neuronal driving force indicator for electrical modulation energy conveyed through each electrode of the plurality based on the respective determined perception threshold; and modify the modulation parameter set based on the modified neuronal driving force indicator for electrical modulation energy conveyed through each electrode of the plurality before delivering the electrical waveform in accordance with the modulation parameter set.
 17. The system of claim 16, wherein the neuronal driving force indicator is an activating function.
 18. The system of claim 16, wherein the perception threshold is determined using patient feedback.
 19. The system of claim 16, wherein the perception threshold is determined by measuring local field potentials at an electrode of the plurality of electrodes.
 20. A method of operating an implantable neuromodulator coupled to an implanted electrode array, comprising: delivering an electrical waveform from the implantable neuromodulator to at least some electrodes in the implanted electrode array in accordance with a modulation parameter set to generate an electric field, wherein: a first component of the electrical field generated using the electrical waveform extends along a spinal cord of the patient and a second component of the electrical field generated using the electrical waveform extends transverse to the spinal cord of the patient; and a gradient across the first component of the electrical field is weaker than a gradient across the second component of the electrical field. 